1. Field of the Invention
The present invention relates generally to air jet helicopters and more particularly to rotor hub assemblies and systems for distributing air to the rotor blades.
2. Description of the Prior Art
One of the most critical problems in the field of air jet helicopters is counter-acting the centrifugal force of the rotating blades when the rotors must be both hollow and lightweight. This problem of centrifugal force also complicates the requirement that the rotors must be rotatable about their longitudinal axes for adjusting their pitch in order to control the flight of the helicopter. A practical and common solution to both problems, adaptable to two-bladed rotors, is the provision of dual tension straps which are comprised of pluralities of flat steel strapping which span the distance between the airfoil portions (blades) of each rotor and are located on either side of the rotor hub. Upper and lower strap guides at the rotor hub level are generally provided on either side of the rotor hub for maintaining the required strap span when the rotors flex upwardly or downwardly. Thus the tension straps are necessarily spaced apart from the longitudinal axis of the two rotors.
Due to the fact that the tension straps are necessarily spaced apart from the longitudinal axes of the rotors, they torsionally resist the required changes in blade pitch for flight control of the helicopter. It follows that the further they are spaced apart from the rotor axes the more they will resist the pitch control systems. The resultant load on the pitch control systems requires that they have more heavy duty parts and are provided with greater mechanical advantages in order to control the flight of an air jet helicopter than what would be required for a comparable conventional craft.
Another problem in the design of air jet helicopters is the need to provide for vertical flapping (up and down) and horizontal dragging (back and forth) of the rotor blades. A common approach is to integrate some form of a universal ball joint through or around which air flow to the rotors is conducted. It is also common in the prior art to locate the universal ball joint in the rotor hub itself in order to minimize the shift in the axis of the tip-path plane (rotor disc) from that of the rotor mast shaft when the helicopter is in forward, rearward or sidewards flight.
Such use of universal ball joints has many drawbacks. They are expensive to manufacture and maintain. Additionally, they usually preclude the use of non-standard bearings, they require wear-resistant materials, and they are heavy structures usually located far above the center of gravity of the helicopter fuselage. Furthermore, when the universal ball joint is located within or integral with the rotor hub, dual tension straps must be further spaced apart to accomodate their size, further increasing the torsion load on the pitch control systems. The tension strap spacing problem may be additionally aggravated by the rotor hub width requirements when either or both air conduits and a universal ball joint is located within or integral with the rotor hub.